Friday, May 21, 2010
Brief "chaser convergence" thoughts, and Missouri cold core tornado setting 5/20/10
Wednesday's massive chaser convergence and careless driving in central Oklahoma has been a huge topic of conversation the past couple days. Shawna and I actually decided not to chase that day because of the expected chaser hordes and narrow severe focus in central Oklahoma, and we both had work to do. Sometimes one asks, "If there's already hundreds of people out there shooting the same storm, what's to learn and what's the point of being out there adding to the dangerous traffic jams?" Wednesday was just such a day. Things don't look to improve in similar future situations, unless someone gets killed in traffic and it is well documented. Shawna and I are getting increasingly picky about going out on "big" days with a small focus drawing hundreds of chasers and weather yahoos to the same spot.
Anyway, stepping away from the situation in Oklahoma, Wednesday 5/19 brought some "cold core" action to the Garden City/Dodge City area of southwest Kansas in the form of "landspouts" along a surface boundary close to the 500 mb low, photographed nicely by Mike Umscheid. And Thursday 5/20 brought a few "cold core" supercell tornado reports to the Sedalia area in Missouri east-southeast of Kansas City (see photo above). I had family visiting in town, so no storm chasing for me or Shawna. But with my interest in such settings, the Thursday setup is worth documenting briefly. Thankfully, as with most "cold core" setups, none of the tornadoes were strong.
The 2nd graphic above shows visible satellite with overlaid surface and 500 mb features at 2145 UTC. Notice the boundary intersection in west-central MO, similar to the "cold core" tornado composite shown in this paper. This can be a favored area of severe focus (enhanced low-level shear and forcing) for tornado development when a midlevel low (strong cold air aloft) is nearby to the west (northeast KS on 5/20) and there is some instability. In this case, the N-S boundary wasn't a dryline, but a subtle wind shift intersecting the warm front southwest of Sedalia at late afternoon, notable on satellite along with some clearing for sun's heating. The 3rd graphic shows selected images from the SPC mesoanalysis at 2200 UTC , including the midlevel low, total CAPE (near 1000 J/kg over west-central MO, but certainly not as impressive as down in Texas), low-level CAPE (a significant maximum near the boundary intersection), and SRH (enhanced northeast of the warm front, but probably under-represented closer to the front in west-central MO). The presence of the 500 mb low in northeast KS and the low-level CAPE maximum over west-central MO near the boundary intersection were probably the biggest "heads-up" flags seen here for tornado potential in what otherwise looked like a fairly benign environment.
The 4th graphic above is a radar image at about 2340 UTC when the tornado in the photo above (likely EF0) was occuring near Sedalia (see arrow for cell location). The RUC analysis at 2300 UTC (last graphic above), as with many "cold core" type settings, showed most of the CAPE below 500 mb, suggesting sizable low-level stretching. The model-derived estimated hodograph (same graphic) suggested good clockwise curvature to the wind profile and more low-level shear than shown on the SPC mesoanalysis during the afternoon. So, while not an easily forecast event, the presence of these ingredients suggested taking rotational signatures on the nearby Pleasant Hill MO radar and spotter reports very seriously. Indeed, the Kansas City area NWS office did a good job jumping on the situation with tornado warnings and statements as the situation developed and evolved prior to the Sedalia tornado, which is what situational awareness is all about.
- Jon Davies 5/21/10